HLSL: Fix #942: Map SV_TargetN to SPV Location N.

Test/hlsl.targetStruct1.frag

@@ -0,0 +1,19 @@
+struct PSInput {
+  float interp;
+  uint no_interp;
+};
+
+struct PSOutput {
+    float4 o1 : SV_TARGET2;
+    float4 o2 : SV_TARGET1;
+};
+
+PSOutput main(PSInput input : INPUT, out float4 po : SV_TARGET0)
+{
+    PSOutput pso;
+    pso.o1 = float4(float(input.no_interp), input.interp, 0, 1);
+    pso.o2 = 1;
+    po = 0;
+
+    return pso;
+}